if...else if...else and Nested if

Decision making is required when we want to execute a code only if a certain condition is satisfied. The if…elif…else statement is used in Python for decision making.

Python if Statement Syntax

if test expression:
    statement(s)

Here, the program evaluates the test expression and will execute statement(s) only if the text expression is True. If the text expression is False, the statement(s) is not executed. In Python, the body of the if statement is indicated by the indentation. Body starts with an indentation and the first unindented line marks the end. Python interprets non-zero values as True. None and 0 are interpreted as False.

Python if Statement Flowchart

Example: Python if Statement

# In this program, user inputs a number.
# If the number is positive, we print an appropriate message

num = float(input("Enter a number: "))
if num > 0:
    print("Positive number")
print("This is always printed")

Output 1

Enter a number: 3
Positive number
This is always printed

Output 2

Enter a number: -1
This is always printed

In the above example, num > 0 is the test expression. The body of if is executed only if this evaluates to True. When user enters 3, test expression is true and body inside body of if is executed. When user enters -1, test expression is false and body inside body of if is skipped. The print() statement falls outside of the if block (unindented). Hence, it is executed regardless of the test expression. We can see this in our two outputs above.

Python if...else 

Syntax of if...else

if test expression:
    Body of if
else:
    Body of else

The  if..else statement evaluates test expression and will execute body of if only when test condition is True. If the condition is False, body of else is executed. Indentation is used to separate the blocks.

Python if..else Flowchart

Example of if...else

# In this program, user input a number
# Program check if the number is positive or negative and display an appropriate message

num = float(input("Enter a number: "))
if num >= 0:
    print("Positive or Zero")
else:
    print("Negative number")

Output 1

Enter a number: 2
Positive or Zero

Output 2

 
Enter a number: -3
Negative number

In the above example, when user enters 2, the test epression is true and body of if is executed and body of else is skipped. When user enters -3, the test expression is false and body of else is executed and body of if is skipped.

Python if...elif...else 

Syntax of if...elif...else

if test expression:
    Body of if
elif test expression:
    Body of elif
else: 
    Body of else

The elif is short for else if. It allows us to check for multiple expressions. If the condition for if is False, it checks the condition of the next elif block and so on. If all the conditions are False, body of else is executed. Only one block among the several if...elif...else blocks is executed according to the condition. A if block can have only one else block. But it can have multiple elif blocks.

Flowchart of if...elif...else

Example of if...elif...else

# In this program, we input a number
# check if the number is positive or
# negative or zero and display
# an appropriate message

num = float(input("Enter a number: "))
if num > 0:
    print("Positive number")
elif num == 0:
    print("Zero")
else:
    print("Negative number")

Output 1

Enter a number: 2
Positive number

Output 2

Enter a number: 0
Zero

Output 3

Enter a number: -2
Negative number

Python Nested if statements

We can have a if...elif...else statement inside another if...elif...else statement. This is called nesting in computer programming. In fact, any number of these statements can be nested inside one another. Indentation is the only way to figure out the level of nesting. This can get confusing, so must be avoided if we can.

Python Nested if Example

# In this program, we input a number
# check if the number is positive or
# negative or zero and display
# an appropriate message
# This time we use nested if

num = float(input("Enter a number: "))
if num >= 0:
    if num == 0:
        print("Zero")
    else:
        print("Positive number")
else:
    print("Negative number")

Output 1

Enter a number: 5
Positive number

Output 2

Enter a number: -1
Negative number

Output 3

Enter a number: 0
Zero